Mortgage Loans Narratives (Details)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Mortgage Loans on Real Estate [Abstract]
Minimum past due days of single-family nonaccrual loans	2 months		2 months
Average term extension of a single-family modified loan	148 months	128 months
Average interest rate reduction of a single-family modified loan	1.81%	2.28%